1. Field of the Invention
This invention relates to a parallel LD/PD module which transmits and receives a plurality of optical signals via a plurality of channels. A parallel communication system transmits optical signals of four, eight, sixteen or, in general, 2m (m=integer) channels via a tape fiber including parallel individual M element fibers (M=2m). A single-mode fiber has a 125 xcexcm cladding diameter. A standard tape fiber arrays individual element fibers at a 250 xcexcm pitch.
This invention claims the priority of Japanese Patent Application No. 2002-35177 filed on Feb. 13, 2002, which is incorporated herein by reference.
A laser diode chip has at least a 300 xcexcm length and a 300 xcexcm width. A parallel LD module should have M laser diodes (LDs). If lightpaths were formed in a parallel LD module at a 250 xcexcm pitch, the virtual module could not allot laser diodes having sides longer than 300 xcexcm at ends of the 250 xcexcm pitch lightpaths.
2. Description of Related Art
{circle around (1)} M. Shishikura, K. Nagatsuma, T. Ido, M. Tokuda, K. Nakahara, E. Nomoto, T. Sudoh and H. Sano, xe2x80x9c10 Gbpsxc3x974-channel parallel LD modulexe2x80x9d, Proceeding of the 2001 Communications Society Conference of IEICE, C-3-50, p160 (2001)
This pointed out problems of multichannel LD modules which contain a plurality of laser diodes. The problems cited were interchannel interference, crosstalk, heating, and fluctuation of properties induced to a multichannel LD module which arranged laser diodes without enough margins. For overcoming the problems, {circle around (1)} proposed a four channel laser diode module with four laterally extending lightwaveguides formed on a silicon bench and four laser diodes placed at ends of the lightwaveguides on the silicon bench. The laser diodes are arranged side by side at a wide pitch. The wide separation reduces crosstalk between an LD and a neighboring LD.
FIG. 16 is a perspective view of the LD module proposed by {circle around (1)}. {circle around (1)} has a (100) orientation single crystal silicon bench 222 (or base), lightwaveguides of an SiO2 type formed upon the silicon bench 222, and laser diodes LDa, LDb, LDc and LDd. The lightwaveguides consist of a lower cladding, a core and an upper cladding. The cores are made of Ge-doped SiO2. The lower and upper claddings are made of SiO2. The four channel module has four lightwaveguides A, B, C and D. Front ends of the lightwaveguides have a spatial period (pitch d) of 250 xcexcm which is equal to a pitch of element fibers in standardized ribbon fibers and MT connectors. The lightwaveguides have width enlarging region. Rear ends of the lightwaveguides have a wide spatial period (pitch D) of 1000 xcexcm (1 mm). The four laser diodes LDa, LDb, LDc and LDd are mounted on a rear end 226 of the silicon bench 222 behind the final ends of the lightwaveguides at a 1000 xcexcm pitch.
The known LD module {circle around (1)} expands the pitch of channels from 250 xcexcm, which is equal to the pitch of ribbon fibers, to 1000 xcexcm which is suitable for mounting the four laser diodes. {circle around (1)} multiplies the channel pitch by four. {circle around (1)} reported that crosstalks between the neighboring laser diodes (LDa-LDb, LDb-LDc and LDc-LDd) were xe2x88x9240 dB at 10 GHz. {circle around (1)} gave wide separation between the neighboring laser diodes for suppressing LDxe2x80x94LD mutual crosstalks. Enlargement of the lightwaveguides requires a long enlarging region on the silicon bench. A full length of the silicon bench should be 15 mm to 20 mm. The long silicon bench results in a large, bulky four-channel LD module.
An arrangement of separated, individual laser diode chips at final ends of plural lightpaths in parallel is unavoidable for high speed parallel signal transmission. Separation of the laser diodes enables the module to freely choose p-type substrate LDs or n-type substrate LDs and to select oscillation frequencies of laser diodes independently. Installment of independent laser diodes enhances freedom of designing.
The known reference {circle around (1)} has only a signal transmitting device but lacks a signal receiving device. {circle around (1)} will require a signal receiving device and another ribbon fiber containing another set of four-channel element fibers for establishing a bi-directional simultaneous optical communications system. {circle around (1)} will allocate a common wavelength (e.g., 1.3 xcexcm) in both upward signals and downward signals. An extension of {circle around (1)}, which contains a separated signal transmitting device and a separated signal receiving device, will not be annoyed by crosstalks between the transmitting portion and the receiving portion. The known reference {circle around (1)} requires two (binary) fibers for a single channel. The extension of {circle around (1)} is a binary fiber type system which needs 2M fibers for M channels. Modules for the binary fiber type {circle around (1)} will be expensive, large-sized modules due to an independent LD device and an independent PD device.
The binary fiber system like {circle around (1)} has, in addition, a drawback of laying 2M fibers between a subscriber (ONU) and a central station. In the case of a four channel binary system, eight (4xc3x972=8) element fibers should be built between the ONU and the station, which raises the cost of constructing the system.
A preferable one is a multichannel system which can exchange four channel signals by four fibers by allotting two different wavelengths (e.g., 1.3 xcexcm and 1.55 xcexcm) to an upward signal stream (from ONU to station) and a downward signal stream (from station to ONU). One purpose of the present invention is to provide a bi-directional simultaneous multichannel LD/PD module which can carry signals up and down by a plurality of fibers whose number is equal to the number of channels. Another purpose of the present invention is to provide a bi-directional simultaneous multichannel LD/PD module of low-cost, small-size and high reliability.
There is a known single-fiber LD/PD module, which aims at simultaneous, bi-directional signal transmission of a single channel, positioning a laser diode, a photodiode and a wavelength selective filter at a point of a y-branch formed upon a lightwaveguide layer on a silicon bench in two dimensions. For example,
{circle around (2)} Japanese Patent Laying Open No.11-68705, xe2x80x9cTwo-way WDM optical transmission reception modulexe2x80x9d proposed a single-channel LD/PD module which has a silicon bench, a y-branched SiO2 lightwaveguide formed on the silicon bench, a laser diode (transmitting 1.3 xcexcm light) deposited at an upper left end of xe2x80x9cyxe2x80x9d, a photodiode (receiving 1.55 xcexcm light) deposited at a bottom end of xe2x80x9cyxe2x80x9d, an end of a fiber fitted at an upper right end of xe2x80x9cyxe2x80x9d, and a WDM (wavelength division multiplexer) at the branch for allowing 1.55 xcexcm light to pass and reflecting 1.3 xcexcm light. On the silicon bench, the 1.3 xcexcm LD beam depicts a V-shaped locus and the 1.55 xcexcm PD beam a /-shaped locus. The known reference {circle around (2)} contrives to reduce electrical crosstalk by positioning the LD and the PD in reverse directions regarding the WDM. Since {circle around (2)} is a module on an ONU (optical network unit; subscriber), a single-channel is sufficient.
An ONU is satisfied with a module having a single LD (1.3 xcexcm) and a single PD (1.55 xcexcm). The relation of the wavelengths is reversed for an ONU and a station. The central station should be equipped with a station module having an LD which emits 1.55 xcexcm and a PD which senses 1.3 xcexcm. The central station may utilize single-channel modules similar to the module of an ONU. The central station should have N single-channel modules for exchange signals with N ONUs. N, which is a number of ONUs, is a very large number. Installment of N single-channel modules would occupy a vast volume in the central station.
Instead of the single-channel modules, multi-channel modules are favorable for the central station for alleviating the space of installing communication modules at the station. Most of the volume of a module is consumed by benches, packages and cases. PDs, LDs and lightpaths are small elements. A multichannel module, for example, a four channel, eight channel, sixteen channel, or thirty-two channel module, would be made to be a small size nearly equal to a single-channel one. A demand of multichannel modules for station modules occurs. An extension of the teaching of the single-channel {circle around (2)} that couples PDs and LDs to fibers by horizontal, planar y-branches may be a candidate of multichannel modules. The virtual extension model may be called a planar type which connects individual sets of a laser diode and a photodiode by a y-branch on a silicon bench, and would consume a huge space for a plurality of y-branches on the silicon bench. The virtual planar M-channel module would be similar to a series of horizontally aligning M single-channel modules. Such a planar type is insignificant for the purpose of reducing size and cost of station communication modules.
If photodiodes (PDs) were provided near laser diodes (LDs) for the sake of reducing the module size, LD/PD access would raise optical crosstalk and electrical crosstalk between the LDs and the PDs. Large optical, electrical crosstalk prohibits the LD modules from transmitting optical signals simultaneously in bilateral directions. An enough distance should be maintained between the PDs and the LDs in a longitudinal direction and in a lateral direction for suppressing the LD/PD crosstalk.
What is the reason why conventional single-channel bi-directional LD/PD modules should require such a wide two-dimensional extension of, for example, {circle around (2)} Japanese Patent Laying Open No.11-68705 which unifies and divides a transmitting beam and a receiving beam by a y-branch horizontally formed on a silicon bench? The reason causing such a wide extension is that the conventional bi-directional modules two-dimensionally divide and unify two different wavelengths (e.g., 1.3 xcexcm and 1.55 xcexcm) on a common level of the silicon bench. Planar, two-dimensional unification or division of two beams causes such a y-branch which forces a silicon bench to consume a wide area.
Area-consumptive y-branches contradict the requirement of producing small-sized LD/PD modules. {circle around (2)}, which is a single-channel LD/PD module which has a single LD and a single PD, may submit to enlargement of size induced by the planar y-branch. Multichannel transmission will urge LD/PD modules to reduce the size.
A single bi-directional LD/PD module has an intrinsic weak point of electrical crosstalk and optical crosstalk between a laser diode and a photodiode. Access of PD/LD increases the crosstalk. Large crosstalk disturbs optical communications. A photodiode should be far separated from a laser diode for reducing the crosstalk in an LD/PD module. For the purpose, the known reference {circle around (2)} positions the photodiode at the bottom end point of xe2x80x9cyxe2x80x9d far distanced from the laser diode laid at a top left end of xe2x80x9cyxe2x80x9d. The separation increases the length of the silicon bench. Allotment of a wide planar distance between a laser diode and a photodiode contradicts the purpose of reducing the size of a module.
Simultaneous exchange of a plurality of signals in two directions imposes the following requirements upon multichannel simultaneous bidirectional LD/PD modules having a plurality of laser diodes and photodiodes.
(1) A sufficient spacing between neighboring laser diodes (LDs).
(2) An enough wide distance between laser diodes (LDs) and photodiodes (PDs).
(3) Low cost, small-size, facile fabrication by planar lightwave circuits.
The present invention proposes a multichannel parallel LD/PD module comprising a bench, a plurality of lightpaths having an initial narrow width region, an intermediate width enlarging region and a final wide width region, a plurality of photodiodes or a photodiode array mounted on the narrow width region, a wavelength selective filter formed on the narrow width region for selectively reflecting receiving beams, and a plurality of light emitting devices fitted behind rear ends of the lightpaths on the bench. Traveling in element fibers from an outer ribbon fiber, receiving signal beams go into the lightpaths via the front ends and are reflected by the wavelength selective filter slantingly upward at the narrow width region. The reflected beams go into the photodiodes and are converted into photocurrents in proportion to the receiving signals. Emanating from the laser diodes, transmitting signal beams enter the lightpaths via the final ends of the wide width region, propagate backward in the enlarging region, arrive at the narrow width region, transfer to outer element fibers in a connector and propagate in the ribbon fiber following the connector.
The multichannel lightpaths are either lightwaveguides or optical fibers. In the case of the optical fibers, a set of the optical fibers should be embedded into curving longitudinal grooves preparatorily perforated on a silicon bench. In the case of the lightwaveguides, cores should be formed by doping a dopant into transparent inorganic SiO2 lightwaveguide layer or an organic polyimide lightwaveguide layer as a cladding.
Smooth expansion of the multichannel lightguides requires a long width enlarging region at an intermediate part of the bench. The narrower initial pitch (spatial period) of the lightpaths is denoted by xe2x80x9cdxe2x80x9d. The wider final pitch of the lightpaths is denoted by xe2x80x9cDxe2x80x9d. An enlarging ratio is given by D/d. A larger ratio D/d demands a longer width enlarging region. The wide space upon the long width enlarging region can be utilized by preparing metallized wirings for photodiodes or installing preamplifiers for amplifying photocurrents of photodiodes.
The transmitting portion can consist of only laser diodes and wirings for the LDs. But in addition, the transmitting portion preferably includes monitoring photodiodes for sensing backward beams from the laser diodes or an LD driving IC for reducing inductance of wirings just behind the laser diodes.
The present invention proposes a multichannel simultaneous bidirectional LD/PD module having the following structure.
(1) The module has a set of multichannel lightpaths (lightwaveguides or optical fibers) which contain an initial narrow width region which has the same pitch as standardized ribbon fibers and connectors, an intermediate curving width enlarging region and a final wide width region for installing light emitting devices (laser diodes or light emitting diodes) with sufficient margins. A enlarging ratio D/d, where d is an initial pitch of the paths in the narrow width region and D is a final pitch of the paths in the wide width region, should be 1.5 to 6. A favorable range of D/d is D/d=2 to D/d=4. A pitch of element fibers in the standardized ribbon fibers is 250 xcexcm. The narrower pitch d should be equal to be 250 xcexcm. The wider pitch D can be D=375 xcexcm to 1500 xcexcm. The curving expanding lightpaths formed upon the bench are either dielectric lightwaveguides or optical fibers. In the case of the lightwaveguides, both inorganic silicon dioxide SiO2 lightwaveguides and organic polyimide lightwaveguides are available.
(2) A wavelength selective filter which selectively reflects only receiving beams from a ribbon fiber slantingly upward is upholstered at an intermediate spot of the initial narrow width region on the bench. Transmitting signal beam frequency is denoted by xcex1. Receiving signal beam frequency is denoted by xcex2. The wavelength selective filter reflects all xcex2 propagating forward from an outer fiber but allows all xcex1 emanated from the laser diodes to pass.
(3) A channel number (M) of photodiodes are mounted halfway on the narrow width region on the bench in front of the wavelength selective filter. Signal beams reflected by the wavelength selective filter upward are sensed by the photodiodes. The photodiodes are either M individual photodiodes or a photodiode array containing M elements. The photodiodes array side by side at the narrower pitch d.
(4) A channel number (M) of light emitting devices (laser diodes LDs or light emitting diodes LEDs) are fitted at ends of the lightpaths. The light emitting devices (LDs, or LEDs) align side by side at the longer pitch D. The light emitting devices generate transmitting signal beams and give the beams to the lightpaths.
(5) The width enlarging region of the lightpaths is immune from both laser diodes and photodiodes. Optionally, metallized wirings can be fabricated for connecting the photodiodes with outer circuits on the width enlarging region of the lightpaths. Alternatively, a preamplifier can be mounted on the width enlarging region for amplifying photocurrents of the photodiodes.
(6) Guidepins or guideholes should be fabricated on forefronts of the module or an outer connector for facilitating coupling of the module to the connector.
(7) Monitoring photodiodes can be installed behind the laser diodes for monitoring the LD power and for maintaining the LD power at a constant level. Otherwise, an LD driving IC can be mounted behind the laser diodes.
(8) An LD/PD module is completed by packaging a half-product containing the bench, the LDs, the PDs and the leadframe. An inexpensive package can be provided by resin moulding. For example, low-cost plastic packages are provided by transfermolding half-products with an epoxy resin in a metallic mould.
The standardized tape fiber pitch (250 xcexcm) is smaller than a side (300 xcexcm to 500 xcexcm) of prevalent square laser diodes. Standardized tape fibers which transmit a plurality of signals have too narrow in pitch to mount the laser diodes on any spot of parallel light guides which can couple with the tape fibers. The present invention provides an LD/PD module with a lightpath width enlarging region for enlarging spacings between lightpaths, a narrow lightpath width region for mounting a plurality of light receiving devices thereon, and a wide lightpath width region for installing a plurality of light emitting devices. The lightpath width enlarging region results in giving room for mounting the light receiving/emitting devices. Thus, the invention has the following effects.
(1) The lightpath width enlarging region enables the present invention to produce low-cost, practical, small-sized parallel LD/PD modules which are suitable for directly coupling to standardized tape fibers having a plurality of fibers in parallel with a predetermined pitch (250 xcexcm pitch).
(2) The present invention makes the best use of the lightpath width enlarging region, which might be seemingly a redundant space, as a space for installing photodiodes and producing wiring patterns for the photodiodes. The space is doubly utilized for expanding a lightpath width and for making receiving portions including the photodiodes, which reduces the size and cost of the module. Optionally, preamplifiers can be mounted near the photodiodes on the width expanding region for killing noise and enhancing receiving sensitivity.
(3) The long, wide lightpath width enlarging region, which might be seemingly a useless space, is effective to alleviate electrical, optical and electromagnetic crosstalk between the laser diodes (LDs) and the photodiodes (PDs) by separating the PDs from the LDs with a long interval. Low crosstalk enhances the sensitivity of the signal receiving part.
(4) The laser diodes are furnished with a margin at final ends of the width enlarged lightwaveguides following the lightpath width enlarging regions. The separated laser diodes are individually mounted at the ends of the paths by a sufficient pitch, which alleviates mutual electrical crosstalk between the laser diodes.
(5) The LD/PD module of the present invention can be coupled to a standardized connector, for example, an MT connector by either attaching guidepins or forming guideholes on a forehead of the module. The module has an interface that satisfies the requirements of the standards of optical communications, which ensures facile utility and wide applicability.
(6) Transfermolding half-products with a resin enables the present invention to obtain low-cost, high-reliability, waterproof packages for mass production.